This invention relates to thermal insulation materials having high fire resistance and low thermal conductivity, and is particularly concerned with resin compositions and composites, particularly polyimide or epoxy resin compositions, and composites formed therewith, incorporating certain additives to substantially increase fire resistance, and which are particularly applicable as structural components, e.g., as an acoustic panel-fire wall structure in aircraft.
Considerable effort and funds have been expended over the past several years in programs to develop the potential of organic composite materials for use in aircraft structures, among others. These studies have shown that the use of high-stiffness, high-strength composites, such as graphite-epoxy, can reduce the weight of structural components by as much as 50% and thus improve structural efficiency while providing significant benefits in cost and performance. The most commonly employed class of resins for this use, depending upon the particular application, are epoxies, polyesters, phenolics and polyimides.
Thus, using graphite-polyimide as the composite, a structural component is available that has good strength, is light-weight, and has some intrinsic fire resistance, in that the polyimide will not readily burn at low temperatures. However, at 2,000.degree. F. flame conditions, the polyimide will decompose to form a char on the flame side. This char, though, is so thin that it will allow heat to get through to the backside and decompose the resin. Thus, this could create a hazard due to the possibility of the decomposition products igniting, thereby generating a fire on the backside, even though the parent polymer e.g. polyimide, does not readily burn. Furthermore, with the resin volatilizing away, the heat transfer through the backside is sufficient to ignite other combustible articles in contact with this fire wall. Therefore, a need exists for a non-burning resin composition that has good stability, is a good char former, and has low thermal conductivity.
There are two problem areas where such a non-burning resin composition can be used on an aircraft: (1) in the engine nacelle, as part of the acoustic panel/fire wall structure, and for this application, polyimides are particularly desirable; and (2) as part of the external surface, where graphite/epoxies are the usual materials of construction.
In regard to this latter case, instances have been reported of the resultant degradation of graphite/epoxy composites due to fire and the consequent breaking up of the graphite fibers and the spreading of these fibers to electrical equipment. Thus, any method that is developed to contain these short conductive fibers and prevent their spreading would be of great value.
Therefore, the use of graphite fiber/resin composites depends not only on the strength of the composite due to the presence of the graphite, but on the fire resistance of the resin, as well. There are many additives that, when incorporated into the resin, will act as fire retardants. Some, such as alumina trihydrate, ammonium phosphate, and zinc borate, are solids that offer excellent fire resistance. The hydrated alumina will offer fire protection by giving off water at a relatively low temperature; however, this temperature is usually around the processing temperature of some resins, e.g., polyimides. Ammonium phosphate and zinc borate are effective at higher temperatures, but, as with the hydrated alumina, these are all solid particulates, and they adversely affect the mechanical properties of the laminate, i.e. cause increase in laminate thickness with a consequent decrease in strength. Many other additives are available that are soluble in the resin, but they are good fire retardants only at relatively low fire temperatures, i.e. around 500.degree. F. to 1,000.degree. F.
There accordingly has arisen the need for a substance which can be incorporated into the resin and which will give protection to the resin at high temperatures, e.g. of the order of 2,000.degree. F., to provide a resin composition which is non-burning and has a low thermal conductivity, and wherein such substance functions as a char stabilizer. However, use of resin-soluble additives for this purpose, such as the reaction product of hydroquinone and phenylphosphonic dichloride, although alleged to be a fire retardant when impregnated into clothing (Ger. Offen. No. 2,236,038, Jan. 31, 1974; Ger. Offen. No. 2,236,039, Jan. 31, 1974; Ger. Offen. No. 2,346,787, Apr. 3, 1975; U.S. Pat. No. 3,853,819; U.S. Pat. No. 3,894,986; U.S. Pat. No. 3,900,444; and U.S. Pat. No. 3,941,752), will not withstand temperatures much above 1,000.degree. F. Thus, for aircraft utilization, whether in the engine nacelle, or on the external skin, where burning fuel fires could result in temperatures around 2,000.degree. F., any organic-soluble additive that can result in a stabilized char upon burning is needed.
It has been known that phosphorus derivatives make good fire retarding agents, as exemplified by the following: U.S. Pat. Nos. 3,941,752; 3,900,444; 3,894,986; 3,853,819; 2,577,281; 2,642,413; 2,716,639; 3,450,677; 3,640,823; 3,685,974; 3,712,789; 3,370,029; and 3,963,437 (among others). Most of the work with fire retardant compositions has been concerned with incorporation of various phosphorylated derivatives into polyurethanes, or fiber forming compositions. Furthermore, in most cases, these compositions have had to be stable to relatively low flame conditions, such as burning matches, cigarettes or burning wood, e.g. up to about 800.degree.-1,000.degree. F. When incorporated into a glass cloth/polyimide resin laminate and burned at 2,000.degree. F. (the FAA requirement for burn-through stability), they did not pass.
However, certain compounds have been used as fire retardants that were found to show excellent fire resistance to a 2,000.degree. F. flame. Notable among these was ammonium phosphate. However, ammonium phosphate is not soluble in the resin, e.g. polyimide, system. Thus, the glass-resin laminate formed from such resin system had poor mechanical properties. Accordingly, a resin-soluble additive is required having the high fire retardant behavior of ammonium phosphate.
Accordingly, one object of the invention is to provide resin compositions and composites having high fire resistance and low thermal conductivity. Another object is the provision of compositions and composites of the above type having utility as a fire barrier, particularly applicable as an aircraft structural component, e.g. an acoustic panel-firewall structure capable of withstanding high temperature, e.g. a 2,000.degree. F. flame temperature. A still further object is the provision of resin compositions, particularly polyimide and epoxy compositions, and composites produced therefrom, such as polyimide-glass fabric or epoxy-glass fabric composites or laminates, having incorporated therein a substance which substantially increases the fire resistance of the resin and reduces its thermal conductivity, substantially without adversely affecting the physical and mechanical properties of the composite, and which functions to stabilize the resin or resin char at high temperatures, e.g. a 2,000.degree. F. flame temperature, and maintains the structural integrity of the composite.